As an image display device substitutable for liquid crystal device (LCD), image display device (a display) with the use of technology such as an electrophoresis method, an electrochromic method, a thermal method, two colors of particle-rotary method is proposed.
As for these image display device, it is conceivable as inexpensive visual display device of the next generation from a merit having wide field of vision close to normal printed matter, having smaller consumption power, and having a memory capability in comparison with LCD, spreading out to a display for portable device, and an electronic paper is expected.
Recently, electrophoresis method is proposed that micro-encapsulate dispersion liquid made up with dispersion particles and coloration solution and dispose the liquid between faced substrates. However, in the electrophoresis method, there is a problem of lacking imaging repetition stability, because particles with high specific gravity of titanium oxide is scattered within solution of low specific gravity, it is easy to subside, difficult to maintain a stability of dispersion state, and difficult in long-term conservation to add dye in solution to render a color. Even in the case of micro-encapsulation, cell size is diminished to a microcapsule level in order to make it hard to appear, however, an essential problem was not overcome at all.
Besides the electrophoresis method using behavior in the solution as above-mentioned, recently, a device for displaying wherein two kinds of toner particles which are different in a color of charging polarity are placed between a pair of substrates, and an electric field is given to fly and fix the particles to the substrates in different direction without using solution is proposed. [The Imaging Society of Japan “Japan Hardcopy'99” (Jul. 21-23, 1999) Transaction Pages 249-252, etc.]
Movement mechanism of such a dry visual display device employs as display device with material mixed two kinds in color and charging polarity are an electrode substrate and apply the voltage to generate an electric field between electrode substrates for flying the charged particle which have different polarity to a different direction.
The above-mentioned dry process image display device using particles behavior in a gas comprising particles and a substrate without using any solution, there are advantage that a problem such as sedimentation and aggregation of the particles in the electrophoresis method is solved and transfer resistance of particles is small to make response speed is faster.
However, with such a dry visual display device, there were the problems as the following:    (1) Structure becomes complicated to dispose the electric charge transportation layer in one part of a substrate, and a layer of charge production, which make it difficult to escape an electric charge from an electroconductive particle constantly, resulting in a lack of stability.    (2) Driving voltage becomes greatly large which make it impossible to move particles unless several hundred volts, comparing to an electrophoresis method of which particle movement was possible at around several tens of volts.    (3) Charge amount of particle itself is a most important parameter in controlling the force generating by an electric field and adhesive force between the particles and an electrode substrate.
However, it is difficult that precision control of charge amount in particle in itself because charge characteristic of particles is influenced by materials of particle itself.
In addition, when it is considered that fine particles as display element, a color toner of the particle is required to be white or black so that contrast becomes clear. On the contrary, as polymer fine particles of general-purpose resin render a clear color, when it became fine particles, it is possible to employ as particles for white by diffused reflection of light. However, to get a black particle, it is necessary to add color and dye of carbon in a polymerization process of the particle which make it very difficult to achieve.    (4) It is necessary to reduce adhesive force between particles or between particles and substrates to move particles stably and repeatedly. For this purpose, the method of coating particles with low adhesive resin is conceivable; however, a crush process is necessary because particles cohere at the time of drying in normal wet processing.    (5) The particles were generally prepared as spherical particles using such method as suspension polymerization or emulsion polymerization dispersing droplets of resin particles into an aqueous disperse medium, and in the case where white particles are prepared, they contains titanium oxide, zinc oxide, silicon oxide as a coloring material. These coloring materials are difficult to be contained uniformly and sufficiently to the aqueous disperse medium because they have heavy specific gravity and higher affinity to water than the droplets.
Additionally, even an indefinite particle of a single resin appears as white by diffused reflection of light at the surface, however, in the image display device of flying and moving particles, a spherical particle is ideal in the viewpoint of fluidity. However, the indefinite particle is hard to fly restricted by a structural disturbance thereby requiring a great driving voltage.    (6) The particles were generally prepared as spherical particles using such method as suspension polymerization or emulsion polymerization dispersing droplets of resin particles into an aqueous disperse medium, however, they must be characterized by either positive or negative by the polarity of the applied voltage so as to determine a flying direction, and enough charge amount is required for achieving highly efficient flight and movement.    (7) In an occasion of forming an electric field, an ideal flight of particles is not realized, and contrast of a display screen is insufficient for displaying favorable images stably.
The object of the present invention is to provide particles for displaying images and image display device having the following characteristic under the situation as above-mentioned, overcoming the problems in dry image display devices.    (a) It is superior in stability and particularly in repetition durability.    (b) It is superior in stability and particularly in memory characteristics.    (c) It is superior in stability, and particularly in adaptive characteristics to a temperature change.    (d) Application of charges to the particle is sufficiently achieved and, when forming an electric field, an ideal flight of particles is realized resulting in displaying favorable images with enough contrast stably.    (e) Aggregation of the particles is prevented enabling to display images superior in stability.    (f) White is distinctly displayed with low driving voltage.    (g) The application of the character of either positive or negative to the particles and maintaining the charge amount are easy, and control of the charge amount is possible.    (h) When forming an electric field, an ideal flight of particles is realized to display favorable images with enough contrast stably.
As a result of repeated zealous study by the inventors of the present invention to achieve the object, they found the following knowledge and based on the knowledge, they completed the present invention.    (i) By preparing the ingredients for the particles coated with resin, the image display devices should be schemed for elevated capability and the durability of the image display device should be improved.    (ii) By preparing the ingredients for the particles with a small Span of the particle diameter distribution, an image with a great contrast ratio should be obtained and durability should be improved.    (iii) By preparing the ingredients for the particles with slow charge attenuation, an image display device superior in stability and, particularly, in memory characteristic should be obtained.    (iv) By preparing the ingredients for the particles regulating the ratio between the surface hardness at the temperature of 0° C. and the surface hardness at the temperature of 100° C., an image display device superior in stability and, particularly, in response to the change of the temperature should be obtained.    (v) By preparing the ingredients for the particles regulating the tensile break strength, an image display device superior in stability and, particularly, in response to the change of the temperature should be obtained.    (vi) By preparing the ingredients for the particles regulating Izod impact strength, an image display device superior in stability and, particularly, in response to the change of the temperature should be obtained.    (vii) By preparing the ingredients for the particles regulating the abrasion loss (Taber), an image display device superior in stability and, particularly, in response to the change of the temperature should be obtained.    (viii) By preparing the ingredients for the particles regulating the tensile elastic modulus, an image display device having capability of displaying the images superior in stability and, particularly, in repetition durability should be obtained.    (ix) By preparing the ingredients for the particles regulating the flexural elastic modulus, an image display device having capability of displaying the images superior in stability and, particularly, in repetition durability should be obtained.    (x) By preparing the ingredients for the particles regulating the tear strength, an image display device having capability of displaying the images superior in stability and, particularly, in repetition durability should be obtained.    (xi) By preparing a group of combined particles comprising mother particles whereon many child particles of at least one kind adhere, an image display device having capability of displaying the images superior in repetition durability at a low driving voltage, cheap, and achieving the compatibility of improving stability and reducing the driving voltage should be obtained.    (xii) By preparing the ingredients for the particles obtained by surface treating fine particles with a solution of charge control agent, attachment of charging ability over the particles should be sufficiently carried out and an ideal flight and movement should be realized in an occasion of forming an electric field, and accordingly, favorable images with sufficient contrast should be stably obtained.    (xiii) By preparing the ingredients for the particles obtained by resin coating them by means of spraying a solution of dissolving resin, an aggregation of the particles should be prevented and favorable images with extended longevity against repeating display and with superior stability should be easily obtained.    (xiv) By preparing the ingredients for the particles at least one resin layer is formed as an outer layer over a spherical central component by coating a resin comprising a component whose index of refraction is different from that of the central component, an image display device having capability of displaying white clearly, quickly responsive, and superior in repetition durability at a low driving voltage should be obtained.    (xv) By preparing the ingredients for the particles involving indefinite particles, around which at least one resin layer is formed by coating a resin comprising a component whose index of refraction is different from that of the indefinite particles An image display device having capability of displaying white clearly, quickly responsive, and superior in repetition durability at a low driving voltage should be obtained.    (xvi) By preparing the ingredients for the particles containing a resin component prepared by polymerizing at least one kind of monomer selected from acrylic monomer, methacrylic monomer and styrenic monomer, an image display device easily determining positive or negative and ensuring surface charge density, capable of charge control by the selection of the monomer or blending ratio, quickly responsive, and superior in repetition durability at a low driving voltage should be obtained.    (xvii) By preparing the ingredients for the particles contained in a mixture obtained by blending at least two kinds of said particles different in both color and charge characteristic, and by settling a difference between each surface charge density within the range of 2 to 150 μC/m2, an ideal flight and movement of particle under the formation of the electric field should be realized and accordingly, favorable images with sufficient contrast and without any unevenness should be stably obtained.
Namely, the present invention provides particles for displaying images used in an image display device which displays images by flying and moving at least one group of particles enclosed between a pair of facing substrates of which at least one is transparent and across the substrates, an electric field being applied, wherein the ingredients for the particles satisfy at least one requirement among the following:    (1) they are coated with a resin;    (2) Span of particle diameter distribution defined by the following equation is less than 5:Span=(d0.9−d0.1)/d0.5wherein d0.1 represents a particle diameter (μm) of the particles whose ratio of particles equal to or less than it is 10%, d0.5 represents a particle diameter (μm) defining that 50% of the particles are greater than this, and another 50% of the particles are smaller than this, d0.9 represents a particle diameter of the particles whose ratio of particles equal to or smaller than it is 90% each in the particle diameter distribution;    (3) in the case where the surfaces of the particles are charged by applying a voltage of 8 kV onto a Corona generator deployed at a distance of 1 mm from the surface of the particles, a surface potential of the particle 0.3 second after the discharge is greater than 300 V;    (4) a ratio between a surface hardness at 0° C. and a surface hardness at 100° C. is 1.7 or smaller;    (5) tensile break strength is 20 MPa or greater;    (6) Izod impact strength (with a notch) is 100 J/m or greater;    (7) abrasion loss (Taber) is 22 mg or less;    (8) tensile elastic modulus is 24.5 MPa or greater;    (9) flexural elastic modulus is 44.1 MPa or greater;    (10) tear strength is 100 kg/cm or greater;    (11) they are a group of combined particles comprising mother particles whereon many child particles of at least one kind adhere;    (12) they are obtained by surface treating fine particles by the use of a solution of charge control agent;    (13) they are coated with a resin by spraying a solution prepared by dissolving resin;    (14) at least one resin layer is formed as an outer layer over a spherical central component by coating a resin comprising a component whose index of refraction is different from that of the central component;    (15) they involve an indefinite particles around which at least one resin layer is formed by coating a resin comprising a component whose index of refraction is different from that of the indefinite particle; or    (16) they contain a resin component prepared by polymerizing at least one kind of monomer selected from acrylic monomer, methacrylic monomer and styrenic monomer.
The present invention also provides an image display device with the use of the particle.